It is known in the art to provide vascular grafts as artificial vascular prostheses to be implanted in individuals with diseased blood vessels. For example, if an individual is suffering from atherosclerosis then a section of blood vessel may be replaced with a vascular graft.
The problem with such vascular grafts is that they have a tendency to cause turbulence in the flow of the blood that they carry, particularly at the join between the vascular graft and the blood vessel at either end. This can result in plaque formation, reduced flow capacity and thromboses in the blood vessel.
WO-A-00/38591 discloses a vascular graft in which a tubular graft is provided with four equally spaced ridges on the interior of the graft. Each edge is in the form of an axially extending helix. The ridges induce helical flow to the blood passing through the vascular graft. The provision of a helical blood flow reduces the turbulence of the blood in the vascular graft which, in turn, reduces the likelihood of plaque formation, reduced flow capacity and thromboses.
WO-A-03/045278 discloses a method for introducing an internal helical formation into a vascular graft. The method comprises supporting a flexible tubular concertinaed material on a mandrel having a groove therein and placing a helical former corresponding to the groove around the material. This deforms the material to have an internal helical formation corresponding to the shape of the groove. The material is set in that configuration and the former and the mandrel can then be removed.
The problem with the method disclosed in WO-A-03/045278 is that, in practice, the method is quite slow and cannot readily form part of a production process.
A problem with vascular grafts, in general, whether or not they have an internal helical formation is that while vascular grafts are usually flexible, they do not exactly replicate the physical properties of the blood vessel that they replace. Normally, the vascular graft will follow a smooth curve when it is bent. However, if it is bent excessively then there is a risk that instead of forming a smooth curve it will form a kink, effectively blocking the cross-section of the vascular graft. If a kink occurs while the vascular graft is implanted then this can be very dangerous as blood flow is significantly reduced. This is particularly a problem if a helical formation is provided on the vascular graft as described in WO-A-03/04278 because the concertinaing of the vascular graft can tend to increase the likelihood of kinking taking place.